A non-aqueous electrolyte lithium ion or lithium secondary battery using a carbon material, an oxide, lithium alloy or lithium metal as an anode is attracting attention as a power source for a mobile phone, a notebook computer and so on because of a high energy density realized thereby.
In this secondary battery, a film referred to as a surface film, a protection film, SEI (Solid Electrolyte Interface), a coating film or the like (hereinafter referred to as “surface film”) is known to be formed on the anode surface. The control of the surface film was known to be indispensable for providing higher performance in the anode because the surface film largely affects a charge/discharge efficiency, a cycle life, etc. The reduction of irreversible capacity is desired in the carbon material and the oxide material, and suppression of the reduction of the charge/discharge efficiency and formation of dendrite (branch-like crystal) is desired in the lithium metal or alloy anode.
A variety of techniques have been proposed for solving these problems. For example, suppression of the formation of dendrite by forming a coating layer consisted of lithium fluoride or the like on the surface of the lithium metal or lithium alloy by utilizing a chemical reaction has been proposed.
Patent Document 1 discloses a technique in which the lithium anode is exposed to an electrolyte containing hydrofluoric acid to react the anode with hydrofluoric acid thereby covering the surface thereof with a lithium fluoride film. The hydrofluoric acid is generated by the reaction between LiPF6 and a slight amount of water. In the meantime, lithium hydroxide or lithium oxide is formed on the surface of the lithium anode through natural oxidation in air. The reaction between them produces a lithium fluoride surface film on the anode surface. However, this lithium fluoride film is formed by the reaction between the electrode interface and the liquid and is liable to be contaminated with a side-reaction component in the surface film and a homogeneous film is hardly obtained. There may be cases where the surface film such as lithium hydroxide and lithium oxide is not homogeneously formed or a part of the lithium is exposed as it is, and in these cases, not only the formation of a homogeneous thin film is difficult but also measure against the reaction between water or hydrogen fluoride and lithium may be required. When the reaction is insufficient, unnecessary compound ingredients other than fluoride may remain and cause disadvantages such as reduction of ionic conductivity. Further, in the method of forming a fluoride layer by utilizing such a chemical reaction on the interface as this, selection range of usable fluoride and electrolyte is restricted and there is a case where a stable surface film can be hardly formed with a good yield.
In Patent Document 2, a mixed gas of argon and hydrogen fluoride and aluminum-lithium alloy are reacted and a surface film made of lithium fluoride is obtained on the anode surface. However, when a surface film is present on the lithium metal surface in advance, especially when a plurality of compounds exist, the reaction is likely to be heterogeneous and may be difficult to homogeneously form a lithium fluoride film. Accordingly, obtaining a lithium secondary battery with excellent cycle performance may be difficult.
Patent Document 3 discloses a technique in which a surface coating film structure having a sodium chloride crystalline structure component as a main component is formed on the surface of a lithium sheet having a homogeneous crystalline structure, that is, preferentially oriented a (100) crystalline plane. By doing this, a homogeneous depositing and dissolving reaction, that is, charge and discharge of the battery, can be allegedly performed, the dendrite deposition of the lithium metal is suppressed and the cycle life of the battery is improved. It is described that the material used for the surface film has preferably a halide of lithium and that a solid solution consisting of at least one compound selected from LiCl, LiBr and LiI, and LiF is preferably used. Specifically, in order to produce the solid solution coating film consisting of at least one compound selected from LiCl, LiBr and LiI, and LiF, an anode for a non-aqueous electrolyte battery is fabricated by dipping a lithium sheet having the preferentially oriented (100) crystalline plane prepared by pressing (rolling) into an electrolyte containing at least one of a chlorine molecule or chlorine ion, a bromine molecule or bromine ion and an iodine molecule or iodine ion, and a fluorine molecule or fluorine ion. In this technique, a rolled lithium metal sheet is used and the lithium sheet is likely to be exposed to atmospheric air, and accordingly, a film originating from moisture tends to be formed on the surface which makes the existence of active points heterogeneous, and therefore, fabricating the intended stable surface film may be difficult so that the effect of suppressing the formation of dendrite cannot be necessarily and sufficiently attained.
Further, Naoi et al., reported the effect of a complex between a lanthanoid transition metal such as europium and an imide anion on the lithium metal anode in the academic presentation at the 68th Conference of Electrochemical Society of Japan (September, 2000; Chiba Institute of Technology; Lecture No.: 2A24) and the 41st Battery Symposium in Japan (November, 2000; Nagoya Congress Center; Lecture No. 1E03). Here, Eu(CF3SO3)3 was further added as an additive to the electrolyte prepared by adding LiN(C2F5SO2)2 as a lithium salt to a mixed solvent of propylene carbonate or ethylene carbonate and 1,2-dimethoxyethane, and a surface film made of Eu[(C2F5SO2]3 complex is formed on the Li metal dipped in the electrolyte. This method has some certain effects on the improvement of the cycle life but further improvement is desired. In addition, because it is indispensable to use a comparatively expensive lithium imide salt such as LiN(C2F5SO2)2 as an electrolyte, and even if a lithium salt (for example, generally LiPF6) other than this and a complex which consists of transition metal and CF3SO3−F3S ion are added, a complex consisting of a transition metal and an imide anion is not formed and the cycle performance is not improved. Furthermore, when a lithium imide salt is used as an electrolyte, resistance of the electrolyte becomes higher as compared with a case where LiPF6 etc. is used, and accordingly improvement is desired in that the internal resistance of a battery increases.
In addition, when a carbon material such as graphite and amorphous carbon which can occlude and release lithium ions is used as anode, a technique concerning the improvement in capacity and charge/discharge efficiency is reported.
Patent Document 4 proposes an anode comprising a carbon material covered with aluminum. This allegedly suppresses reduction decomposition of the solvent molecules which solvate lithium ion on the carbon surface and degradation of the cycle life. However, because aluminum may react with a slight amount of water, improvement is desired in that capacity decreases quickly when the cycle is repeated.
Patent Document 5 presents an anode in which the surface of a carbon material is covered with a thin film of a lithium ion conductive electrolyte. It is stated that the decomposition of the solvent produced when the carbon material is used is controlled by doing this, and a lithium-ion secondary battery in which propylene carbonate can be used can be especially provided. However, cracks produced in the solid electrolyte by the stress change at the time of insertion and release of lithium ions may cause degradation of properties. Further, homogeneous reaction on the anode surface may not be attained due to heterogeneity such as a crystal defect of the solid electrolyte, and may lead to degradation of the cycle life.
Patent Document 6 discloses a secondary battery having an anode consisted of a material containing graphite and comprising as the electrolyte a cyclic carbonate and a linear carbonate as the main ingredients and further containing 0.1 wt % to 4 wt % of 1,3-propanesultone and/or 1,4-butanesultone in the electrolyte. Here, it is considered that 1,3-propanesultone and 1,4-butanesultone contribute to the formation of passive film on the carbon material surface, cover an active and highly crystallized carbon material such as natural graphite and artificial graphite with a passive film and exhibit an effect of suppressing decomposition of the electrolyte without spoiling the normal reaction of the battery. However, no excellent effect is obtained in this method, and improvement is desired in that the electric charge due to decomposition of the solvent molecule and anion appears as an irreversible capacity component, which leads to decrease in the first time charging and discharging efficiency. Improvement is also desired in that resistance of the generated film ingredient is high, and especially increasing rate of resistance with the time course under a high temperature is large.
Patent Document 7 discloses a non-aqueous secondary battery having a cathode consisting of a 4 V class active material and an anode comprising a substance having peaks at 55.0 eV and 168.6 eV by XPS analysis on the surface. It is described here that the peak at 55.0 eV is assigned to a lithium sulfur compound and a peak at 168.6 eV forms a film having a SO2 bond, and that although the film having a SO2 bond is stable, it has ion conductivity and it has an action to suppress the decomposition of the electrolyte. However, improvement is desired in this method in that when stored at a high temperature, resistance increases and decrease of the output and deterioration in the capacity of the battery take place.
Patent Document 1: Japanese Patent Laid-Open No. 7-302617.
Patent Document 2: Japanese Patent Laid-Open No. 8-250108.
Patent Document 3: Japanese Patent Laid-Open No. 11-288706.
Patent Document 4: Japanese Patent Laid-Open No. 5-234583.
Patent Document 5: Japanese Patent Laid-Open No. 5-275077.
Patent Document 6: Japanese Patent Laid-Open No. 2000-3724.
Patent Document 7: Japanese Patent Laid-Open No. 2000-323124.